(1) Field of the Invention
The present invention generally relates to a hydraulic brake control apparatus, and more particularly to a hydraulic brake control apparatus which controls a brake fluid pressure in a brake system of an automotive vehicle.
(2) Description of the Related Art
A hydraulic brake control apparatus having two pressure supplying devices connected to wheel cylinders of an automotive brake system is known. For example, Japanese Laid-Open Patent Application No.4-243655 discloses this type of apparatus.
In the conventional apparatus of the above-mentioned publication, a master cylinder supplies a brake fluid pressure responsive to a brake operating force, and a pressure supplying device supplies a brake fluid pressure responsive to a drive signal. The pressure supplying device is controlled such that the brake fluid pressure supplied by the pressure supplying device is set at a brake fluid pressure which is equivalent to the brake operating force multiplied by a predetermined constant.
In the apparatus of the above-mentioned publication, the master cylinder is connected to the wheel cylinders via a switching valve, and the pressure supplying device is connected to the wheel cylinders via the switching valve. When the pressure supplying device normally operates, the flow of the brake fluid from the master cylinder to the wheel cylinders is inhibited by the switching valve, and the brake fluid pressure from the pressure supplying device is supplied to the wheel cylinders.
On the other hand, when a defect in the pressure supplying device has occurred, the flow of the brake fluid from the pressure supplying device to the wheel cylinders is inhibited by the switching valve, and the flow of the brake fluid from the master cylinder to the wheel cylinders is allowed by the switching valve. In this case, the brake fluid pressure responsive to the brake operating force is supplied to the wheel cylinders by the master cylinder.
According to the conventional apparatus of the above-mentioned publication, it is possible to supply a proper brake fluid pressure from the pressure supplying device to the wheel cylinders when the pressure supplying device normally operates, and it is possible to supply a proper brake fluid pressure from the master cylinder to the wheel cylinders when a defect in the pressure supplying device has occurred. Therefore, the conventional apparatus provides a fail-safe function against a defect in the pressure supplying device.
However, if a defect in the switching valve has occurred and the defective valve is fixed at the closed position to close the related brake fluid path, the flow of the brake fluid from the master cylinder to the wheel cylinders is unsuitably inhibited by the defective valve. Hereinafter, this defect will be called the closed-position defect. If the closed-position defect in the switching valve has occurred, it is no longer possible for the conventional apparatus to supply a proper brake fluid pressure from the master cylinder to the wheel cylinders through the switching valve when a defect in the pressure supplying device has occurred.
Therefore, the conventional apparatus of the above-mentioned publication cannot provide the fail-safe function against a defect in the pressure supplying device if the closed-position defect in the switching valve has occurred. In such a case, the conventional apparatus cannot produce a proper brake fluid pressure in the wheel cylinders, and the wheel cylinders cannot produce an adequately great braking force on the vehicle wheels.